Regarding a lithium-ion secondary battery, JP 2004-253174 A, for example, discloses the use of, as a positive electrode active material, a layered lithium-transition metal oxide comprising a hollow particle having an outer shell portion and a space portion inside the outer shell portion. The lithium-transition metal compound oxide proposed therein is such that, when it is cross-sectioned, the proportion of the area of the space portion with respect to the total of the areas of the outer shell portion and the space portion is greater than 0% but less than 20%. The publication states that the use of such a lithium-transition metal oxide for the positive electrode active material makes it possible to provide a non-aqueous electrolyte secondary battery that shows excellent battery performance even under more severe use environments.
The manufacturing method of the layered lithium-transition metal oxide is disclosed in paragraphs 0026 to 0042 of the publication. The manufacturing method disclosed therein roughly includes the following procedures. First, an aqueous solution containing cobalt ions and nickel ions in a predetermined composition ratio is dropped in pure water that is being agitated. Next, sodium hydroxide is dropped therein so that the pH will be 8 to 11, and cobalt and nickel are coprecipitated at a temperature of 40° C. to 80° C. at a number of revolution of 500 rpm to 1500 rpm, to obtain a coprecipitated substance. Next, the obtained coprecipitated substance is filtered, washed with water, and thereafter dried, and then mixed with lithium hydroxide. The mixture is then baked at a temperature of 650° C. to 1100° C. for 1 hour to 24 hours in an atmosphere in which the oxygen partial pressure is controlled, to synthesize a lithium-transition metal oxide.
JP 2011-119092 A discloses active material particles for a lithium secondary battery. The active material particles disclosed therein constitute a hollow structure having a secondary particle, in which a plurality of primary particles of a lithium-transition metal oxide are aggregated, and a hollow portion formed therein. It is also proposed that the secondary particle has a through-hole penetrating from the outside into the hollow portion, and that the BET specific surface area thereof is set to from 0.5 m2/g to 1.9 m2/g. The publication states that such a hollow active material particle can achieve an improvement in high-rate performance, an improvement in durability, prevention of resistance increase, and an improvement in capacity retention ratio at the same time.
JP 3032757 B discloses the use of, as a positive electrode active material, a composite oxide represented by the general formula LixM1−yAyFzO2n−z, (where 0.9≦x≦1.1, 0≦y≦0.5, 0≦z≦0.25, 1≦n≦2, M is at least one transition element selected from the group consisting of Co, Ni, and Mn, and A is at least one element selected from the group consisting of Co, Ni, Mn, B, and Al), although it is unclear whether the positive electrode active material contains such a hollow particle as described above. The just-mentioned patent states that the positive electrode active material forms secondary particles each made of primary particles having a crystal structure with C-axis orientation tendency, and that the ratio (D50/r) of the particle size D50, at which the cumulative volume of the secondary particles reaches 50% in particle size distribution, to the average shorter axis length r of the primary particles is 10≦(D50/r)≦50. According to the patent, it provides a non-aqueous electrolyte secondary battery that shows high discharge voltage, excellent charge-discharge characteristics at high current, and excellent cycle performance.